1) Field of the Invention
The invention relates to a method for sootblowing a recovery boiler, wherein sootblowers of the recovery boiler are divided into sootblowing groups.
The invention further relates to an apparatus for sootblowing a recovery boiler, the apparatus comprising sootblowers arranged in sootblowing groups in the recovery boiler, and a control apparatus.
2) Description of Related Art
In pulp mills, black liquor developed in the course of pulp making is burned in a recovery boiler in order to recover recyclable chemicals and the energy of combustible materials contained in the black liquor. In the recovery boiler, heat is recovered utilizing water tubes that constitute the walls of the recovery boiler and other heat transfer surfaces. Such heat transfer surfaces include superheaters located in a combustion chamber of the recovery boiler and feed water preheaters and boiler banks located in a flue gas passage after the boiler.
When black liquor is burned, considerable amounts of gases, particles, carry over drops and other such combustion by-products emerge that flow through the recovery boiler and the flue gas passage together with combustion gases. Some of the combustion by-products adhere to the heat transfer surfaces, which are thus fouled. Fouling reduces the efficiency of the recovery boiler, since dirt works as an insulating material between the combustion gases and the water to be heated flowing in the tube systems, and steam. In addition, eventually fouling causes clogging, and in order to remove the clogging, the burning process in the recovery boiler has to be stopped. A clogged recovery boiler typically means at least a twenty-four-hour shutdown for the entire production unit, which causes great economic losses for the entire pulp mill.
The heat transfer surfaces of the recovery boiler are sootblown in order to prevent or delay fouling. How often a recovery boiler needs to be sootblown substantially depends on the structure and conditions in the combustion chamber of the recovery boiler, which affect the amount and characteristics of the combustion by-products. It is to be noted that hereinafter in the present invention, unless otherwise indicated, sootblowing a recovery boiler refers to sootblowing both the actual recovery boiler part and the subsequent flue gas passage. Sootblowing is usually carried out using steam, the steam consumption of a sootblowing procedure typically being 4-5 kg/s, which corresponds to about 4-5% of the steam production of the entire recovery boiler; the sootblowing procedure thus consumes a considerably large amount of thermal energy.
At its simplest, sootblowing is a procedure known as sequence sootblowing, wherein sootblowers operate at determined intervals in an order determined by a certain predetermined list. The sootblowing procedure runs at its own pace according to the list, irrespective of whether sootblowing is needed or not, which means that clogging cannot necessarily be prevented even if the sootblowing procedure consumes a high amount of steam.
U.S. Pat. No. 4,718,376 discloses a method comprising assigning the sootblowers into a number of groups, each sootblower being provided with a weight factor which is a percentage of the total time of a sootblowing cycle and which determines the number of sootblowing cycles in which the particular sootblower participates; a sootblowing cycle is the time the sootblowing procedure takes to cover the entire recovery boiler. The weight factor can be modified using data measured from the procedure, such as draft loss increase and heat transfer factor. However, the method is not necessarily fast enough to prevent clogging in some part of the recovery boiler since the sootblowing cycle has to proceed in a predetermined order before a particular part becomes sootblown; therefore, there will be enough time for ash to harden onto the surface of the recovery boiler, after which it is impossible to remove it by sootblowing.
An object of the present invention is to provide a sootblowing method and a sootblowing arrangement that enable the above-mentioned drawbacks to be avoided.
A sootblowing method of the invention is characterized by producing a fouling index for each sootblowing group of the recovery boiler, determining sootblower-specific sootblowing intervals, calculating relative frequency values of the sootblowing groups, selecting, for sootblowing, the sootblowing group and the sootblower such that the sootblowing takes place substantially according to the relative frequency values and the sootblower-specific sootblowing intervals.
A sootblowing apparatus of the invention is characterized in that the control apparatus is arranged to determine sootblower-specific sootblowing intervals, produce a fouling index for each sootblowing group and calculate relative frequency values for the sootblowing groups, select, for sootblowing, the sootblowing group and the sootblower such that the sootblowing takes place substantially according to the relative frequency values and the sootblower-specific sootblowing intervals.
The basic idea of the invention comprises determining sootblower-specific sootblowing intervals; producing a fouling index for each sootblowing group of the recovery boiler to describe the susceptibility to fouling of a part of a particular recovery boiler and correcting the sootblowing interval of the sootblowing groups by utilizing the fouling index; calculating the relative frequency values of the sootblowing group to describe the relative proportion of the sootblowing time of each sootblowing group from the sum of the sootblowing times of all sootblowing groups of the particular recovery boiler; selecting, for the sootblowing procedure, the sootblowing group and the sootblower such that the sootblowing procedure substantially takes place according to the relative frequency values and the sootblower-specific sootblowing intervals. Furthermore, the idea underlying a preferred embodiment comprises producing importance counters for each sootblowing group, the value of the importance counters being increased by the relative frequency value of the sootblowing group after each sootblowing procedure, and, in addition, if the sootblowing procedure has taken place in a sootblowing group of its own, reducing the value of the importance counter by one; selecting, for the sootblowing procedure, the sootblower whose sootblowing time, i.e. the time passed from the start of its previous sootblowing procedure, most exceeds the desired sootblowing interval or whose sootblowing time from its previous sootblowing procedure comes closest to the sootblowing interval calculated for the particular sootblower, and which sootblower belongs to the sootblowing group having the highest importance counter value. Furthermore, the idea of a second preferred embodiment comprises correcting the sootblowing interval of the sootblower by applying the fouling index and the formula
sootblowing interval=starting intervalxe2x88x92fouling indexxc3x97maximum correction 
Furthermore, the idea of a third preferred embodiment comprises adjusting the starting interval of the sootblowers such that the sootblowing interval of the most important sootblowers of the sootblowing group becomes shorter and the starting interval of the less important sootblowers becomes longer while the total time used by the sootblowing group remains unchanged. The idea of yet a fourth preferred embodiment comprises determining the fouling index by fuzzy logic.
An advantage of the invention is that it enables the sootblowing resources to be targeted at critical spots in the boiler for maximum benefit. The recovery boiler is sootblown as evenly as possible, leaving no recovery boiler part unblown for too long, thus preventing ash from hardening. In addition, the conditions in the recovery boiler enable optimal consumption of sootblowing steam.
In the present application, the term xe2x80x98sootblowerxe2x80x99 may also refer to a pair of sootblowers.